The iPhone blog reports on a small, but pretty serious issue with iPhone 3.0:

Matt Janssen has discovered what looks like a bug in iPhone 3.0 Spotlight, whereby the search feature keeps emails in the index/cache even after they’ve been deleted from the inbox and purged from the trash (and even obliterated from the host server). Janssen claims in the video above he’s found emails he deleted several months ago using this method.

UPDATE Tested with both POP3 and IMAP accounts and was not able to reproduce the bug